Add a section to the CORS guide about CrossOrigin annotation.

[wetted]

closes #1271

[wetted]

@sdelamo I wasn't sure how I could include the hello-base guide but without the controller, so I had to make changes that seemed unnecessarily complicated, such that the RunTestScript would pass, and the @CrossOrigin annotation would be included in the solution at the same time. So maybe this is a little too fussy.